In butterfly valves, translational movement of the valve plate relative to the pivot mechanism can be used to effect dynamic torque balancing, and to facilitate actuation by using the torque exerted by fluid on the plate to rotate the latter. To applicant's knowledge, these principles were first set forth in the above-referenced patent application Ser. No. 374,897, filed on 6/30/89. They are schematically illustrated in FIGS. 12A-12C of the present application, wherein numerals 20, 22, 24, 26, and 28 indicate a valve plate, a pivot shaft, a duct, a bracket intersecuring the shaft and plate, and the direction of flow respectively. Arrow 30 and numeral 32 indicate the center of pressure associated with the forces exerted by fluid on the plate 20, and the pivot line about which the plate is rotatable.
In FIG. 12A, the shaft 22 is centered in relation to the longitudinal dimension of the plate 20 and the plate is in a closed position The center of pressure 30 is aligned with the pivot line 32, so the net torque exerted by the fluid on the plate 20 is zero. If the plate 20 is rotated to an open position as illustrated in FIG. 12B, and translated relative to the shaft 22 so that the center of pressure 30 is aligned with the pivot line 32 as illustrated in FIG. 12C, then the plate is effectively torque-balanced and the flow rate associated with the rotational angle 34 can be maintained with minimal input force required from the actuator 36. If the translational movement is too limited in range to achieve torque balancing at all rotational positions of the plate 20, it can still be used to advantage in minimizing the net torque exerted by the fluid, thus still minimizing the input force required from the actuator 36.
If it is desired to change the rotational angle 34, then the plate 20 can be translated relative to the shaft 22 in the appropriate direction to effect a misalignment of the center of pressure 30 with the pivot line 32, thus increasing the net fluid dynamic torque, and the force required to effect the change is exerted in whole or in part by the fluid conveyed in the duct 24.
Butterfly valves that are designed to provide for translational movement of the plate 20 relative to the shaft 22, for either or both of the above-described purposes, are referred to herein as "balanced-torque" butterfly valves.
The balanced-torque butterfly valves illustrated in FIGS. 12A-12C may have disadvantages in some applications. Since the bracket 26 and shaft 22 protrude from the plate 20, the structure as a whole is not well streamlined from a fluid dynamics perspective. This may result in application-dependent problems which are believed to be attributable to the effect of the shaft 22 and bracket 26 as flow restrictors. For example, in tests conducted for balanced-torque butterfly valves which have the shaft 22 positioned on the downstream side of the plate 20, the plate maintained a stable equilibrium position when partially open, but stability was dependent on pressure ratio (i.e. inlet/outlet air pressure). Above a critical pressure ratio, the plate 20 would latch open (i.e. move suddenly to a rotational angle 34 of about ninety degrees). When the valve was tested with the shaft 22 on the upstream side of the plate 20 as indicated in FIGS. 12, considerable stability was observed for rotational angles 34 ranging from zero to slightly in excess of sixty degrees Attempts to rotate the plate 20 out of an equilibrium position (i.e. a position at which the plate is translated relative to the shaft such that the center of pressure 30 is substantially aligned with the pivot line 32) were met with considerable aerodynamic torque resistance.
The current thinking is that the overall exterior shape of the valve plate 20 is the important variable in controlling the latching phenomenon, whereas resistance to rotational movement away from an equilibrium position is best controlled by translationally moving the plate out of the equilibrium position in order to facilitate rotation. In one aspect of the present invention, the latching problem is simplified by eliminating the fluid dynamic effects of the shaft 22 and the bracket 26.
Another disadvantage of previously described balanced-torque butterfly valves is that they sometimes are not well suited for providing feedback regarding the rotational position of the valve plate. That is, it is difficult to sense the rotational position of the valve plate from a location outside the duct unless the pivot shaft is in fixed rotational relationship with the valve plate. In addition, it may be desirable in many applications to provide for both rotational and translational movement of the valve plate via external actuation. A second aspect of the invention resides in the provision of a pivot mechanism which may be adapted to provide for both types of movement via external actuation, while simultaneously providing a ready means for determining the rotational position of the valve plate.
Another disadvantage relates to power transmission between the pivot shaft 22 and the plate 20. One method for translationally moving the plate 20 is to provide rack and pinion gears on the plate and shaft, respectively. However, when the plate 20 is subjected to pressure exerted thereon by the fluid conveyed in the flow path, the gears bear substantially the entire load. This may result in binding, and will almost certainly result in accelerated wear. A third aspect of the invention pertains to minimizing the load-bearing requirements of such power transmission components.